Error while trying to download ios 16.5.1

This could be caused by several things, including not having a good Wi-Fi connection, not having enough storage space on the phone, needing to be connected to power while updating and more. Please check this support page from Apple:

If your iPhone or iPad won't update - Apple Support

Please let us know which iPhone you have and what the errors say. We’d like to help if we can but we need more information.

What happens when you try to update? If there an error message?